Lenders perceive that you are risky, so you must pay 12 percent annual interest to borrow from one of them. You only receive 6 percent on funds you have deposited in the bank. Do the opportunity costs of borrowing and using your own funds differ in this example?

Most restaurant customers tip according to a percentage rule-between 15 and 25 percent of the bill. Diners who have dinner and a $20 bottle of wine usually pay the same percentage of the bottle price as diners who order a $100 bottle. Why, when the same efforts must be made to uncork and pour both bottles?